Ba19Cr12O45: A High Pressure Chromate with an Original Structure

Article pubs.acs.org/IC

Ba19Cr12O45: A High Pressure Chromate with an Original Structure Solved by Electron Diffraction Tomography and Powder X‑ray Diffraction Christophe Lepoittevin,*,†,‡ Justin Jeanneau,†,‡ Pierre Toulemonde,†,‡ André Sulpice,†,‡ and Manuel Núñez-Regueiro†,‡ †

Université Grenoble-Alpes, Institut Néel, F-38000 Grenoble, France CNRS, Institut Néel, F-38000 Grenoble, France

‡

ABSTRACT: We report on the discovery of a Ba-based chromate obtained by high pressure−high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.

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INTRODUCTION After 30 years of research on the superconducting cuprates,1,2 the discovery of high Tc superconductivity in related iron based pnictides in 20083 has stimulated the search of new superconductors. In particular, antiferromagnetic (AFM) systems with 3d transition elements, with a high TNéel temperature, moderate magnetic moments, and crystallographic two-dimensional (2D) layers could be adequate parent phases for new unconventional superconductors. Among 3d elements, chromium has recently focused attention. In 2014 superconductivity was found for the first time in a Cr-based compound, the helimagnet CrAs which superconducts below Tc ≈ 2 K under a moderate pressure of ∼0.8 GPa.4,5 Soon afterward, superconductivity was discovered at ambient pressure in a family of Cr-based one-dimensional (1D) compounds A2Cr3As3 (A = K, Rb, Cs) with Tc values in the 2.2−6.1 K range.6 During these past few years, we have become interested in layered Cr-based oxides, with a particular attention to the Sr-based Ruddlesden−Popper (RP) series Srn+1CrnO3n+1 obtained under high-pressure and high temperature (HP-HT) conditions.7,8 We have used high pressure in order to tune and weaken the AFM order and improve the electrical conductivity of the different members of the series. Up to 20 GPa no superconductivity was found in n = 1 (Sr2CrO4), n = 2 Sr3Cr2O7, and n = ∞ (SrCrO3) members, and the chromates remain insulating.9 We have searched for new layered chromates as probable candidates to become superconductors, by changing Cr−Cr interactions by doping or pressure. In particular, we have © 2017 American Chemical Society

synthesized under HP-HT the ones that are obtained by substitution of Sr2+ with other alkaline earth elements, in order to study the cation size effect. In Sr2CrO4 substitution with a smaller cation Ca2+ led to an isostructural RP Sr-based n = 1 phase and to a weakening of both the insulating behavior and AFM ordering. We have also tried the synthesis of the n = 1 member fully substituted with a bigger cation, i.e., with Ba2+: Ba2CrO4. In our HP-HT conditions, this substitution did not lead to a RP layered phase, as expected, but to a new original cubic phase with the chemical composition Ba19Cr12O45. The current paper is devoted to the synthesis, the crystal structure resolution by combination of electron diffraction tomography (EDT) and powder X-ray diffraction (PXRD), and the physical properties of this new chromate. EDT, a method developed by Kolb et al. in 2007,10 is now commonly used for ab initio structure determination and has proved to be very efficient. Numerous structures, even those presenting complex commensurate11 or incommensurate12 modulations, have been successfully solved by this method these past few years. Namely, Li in Mn/Fe phosphates cathode materials13 or very recently hydrogen positions in single nanocrystals,14 as the detection of light elements is also possible. EDT was an essential tool to solve the crystallographic structure of our new high pressure phase. Received: February 22, 2017 Published: May 24, 2017 6404

DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409

Inorganic Chemistry

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EXPERIMENTAL SECTION

Article

RESULTS AND DISCUSSION Figure 2 shows a possible indexation of the XRD pattern of the sample obtained after its HP-HT treatment with the tetragonal

Synthesis of the new phase Ba19Cr12O45 was achieved by compressing and heating the ambient pressure orthorhombic phase Ba2CrO4 (Pnma, space group No. 62).15 This chromate precursor was obtained using polycrystalline powders of BaCO3 (Sigma-Aldrich, 99%) and Cr2O3 (Sigma-Aldrich 99.9%) as starting materials (Figure 1). The powders

Figure 2. XRD pattern of the Ba2CrO4 sample after its HP-HT treatment with the experimental pattern (in red), the calculated one by Rietveld refinement (in black) and the difference (in blue). Bragg reflections are assigned to a n = 1 Ruddlesden−Popper phase type (I4/ mmm).

Figure 1. XRD pattern of the orthorhombic Ba2CrO4 ambient pressure phase (Pnma) with in red the experimental pattern, in black the calculated one (by Rietveld refinement), and in blue the difference.

were mixed in stoichiometric proportions and pelleted to achieve a homogeneous mixture and then heated at 1000 °C for 12 h in an argon flow. The sample was then grounded and pressed within a gold capsule for high pressure and high temperature synthesis. This capsule was placed in a calcite tube to be electrically insulated from the graphite heater. This assembly, maintained between calcite caps and inserted in the center of a pyrophyllite cell, used as a pressure transmitting medium, was compressed at 6 GPa between two tungsten carbide anvils in the Belt-type press. Then, the Ba2CrO4 sample was heated at 1000 °C for 30 min, quenched down to room temperature, and slowly depressurized. PXRD patterns were collected at room temperature using a Siemens D-5000T diffractometer working in Bragg−Brentano geometry at the Cu Kα wavelength (λα = 1.5406 Å) from 2θ = 10° to 90° with a 0.032° step size. Data were refined with the Rietveld method16 implemented in the Fullprof suite program.17 The magnetic properties were measured in a Métronique Ingénierie SQUID (sensitivity 10−7emu at low field and 10−5 emu at high field). For the transmission electron microscopy (TEM) analysis, the specimen was prepared by crushing a small piece of sample in an agate mortar containing ethanol, and a drop of the suspension was deposited on a copper grid covered with a holey carbon film. TEM imaging was carried out with a Philips CM300ST microscope (LaB6, 300 kV) equipped with a nanoMEGAS precession device Spinningstar, a F416 TVIPS camera, and a Brüker Silicon Drift energy dispersive X-ray (EDX) spectroscopy detector. Manual electron diffraction (ED) tomography was performed using a single tilt tomography sample holder with a +51°/−53° tilt range. The data set was collected by recording an ED pattern in every 1° rotation of the sample holder. A precession angle of 1.2° was applied to integrate the whole intensity of the reciprocal space rods. The data processing was performed using PETS program:18 PETS performed peak hunting on each ED pattern and refined the azimuthal angle between the horizontal axis and the projection of the tilt axis. Then a difference vector space analysis was computed to produce a complete representation of the reciprocal space. The program JANA2006 was used as a graphical interface to find the unit cell and to refine the orientation matrix that allows integrating the reflection intensities of each ED pattern by PETS. Intensities belonging to the same reflection on adjacent patterns were integrated together, and a final list of intensities containing one value per each hkl indices with the corresponding estimated standard deviation was created.19 The structure was solved by the charge flipping algorithm using the program Superflip20 in the computing system JANA2006.21

Figure 3. (a) [110] and (b) [111] SAED patterns of the cubic barium chromate.

lattice (I4/mmm) expected for the n = 1 RP member. The corresponding Rietveld refinement gives the cell parameters a = 4.44 Å and c = 13.31 Å. At this stage, we note that these parameters do not follow the regular decrease expected from the comparison of the alkaline earth ionic radii in coordination number 8: Ba2+ 1.42 Å, Sr2+ 1.26 Å (∼11% smaller), Ca2+ 1.12 Å (∼11% smaller).24 For instance, in this analysis, the a-axis of Ba2CrO4 is found curiously ∼14% larger than the one of Sr2CrO4 (a = 3.81 Å and c = 12.51 Å), while the a-axis of Ca2CrO4 (a = 3.67 Å and c = 12.14 Å)9,22 is less than 4% smaller than the one of Sr2CrO4. Then, if one continues on the basis of this indexation, the n = 1 RP Ba2CrO4 phase can be interpreted as the main phase, and the remaining Bragg reflections are attributed to secondary phases, which at a first sight, do not fit the precursors or any phase present in the crystallographic database. In order to test this interpretation and identify the nonindexed peaks on the XRD pattern, the sample was investigated using TEM. The EDX analysis was carried out on 10 different crystallites of the sample, and it revealed a first major phase with an average ratio Cr/Ba ≈ 0.55 ± 0.15, and a secondary phase containing only Ba. The averaged Cr/Ba ratio of the main phase must be considered with caution since its large standard deviation is an indication of the contribution of the secondary fluorescence of Ba, making Ba content overestimated, which is more or less 6405

DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409

Article

Inorganic Chemistry

Figure 4. (a) hk0, (b) hk1, and (c) hhl selected sections of the reciprocal space indexed in the I-type lattice of the cubic barium chromate.

Figure 5. (a) [100] Electrostatic potential isosurface of the structure solution and (b) the [100] raw structural model for the cubic barium chromate.

Table 1. (a) Ba/Cr−O Distances Obtained from the Raw Structural Model Calculated by SIR2011 Atom1

Atom2

d (Å)

Ba1

O2(x1) O1(x4) O1(x2) O3(x4) O2(x4) O1(x4) O1(x6) O2(x3) O1(x3) O2(x12)

2.81 2.83 2.92 2.41 2.81 2.84 2.00 1.88 2.10 2.85

Ba2

Ba3 Cr1 Cr2

important depending on the analyzed particles thickness. Selected area electron diffraction (SAED) patterns were recorded on the first phase by tilting around crystallographic axes to reconstruct the reciprocal space. This reconstruction evidenced a cubic unit cell, with the cell parameter a ≈ 13.5 Å. The systematic extinction conditions observed on hkl reflections are consistent with a I-type lattice. Figure 3a,b shows the two zone axes [110] and [111] separated by 35°, which is characteristic of a cubic system. A reconstruction of the reciprocal space of the Ba-based secondary phase highlighted a hexagonal unit cell with the parameters a ≈ 10.5 Å and c ≈ 6.5 Å and extinction conditions consistent with the P63/mmc space group. From these TEM results, we can conclude that the initial indexation of the powder XRD pattern using the n = 1 RP lattice was incorrect, and then motivated us to fully determine a structural model of this new Ba-based cubic chromate. From our EDT data set, the averaged unit cell parameters for the cubic symmetry were found to be a = 13.43(8) and α =

Figure 6. Observed Fourier map section at z = 0, with the supplementary oxygen atoms O3 and O4 (in red) deduced by difference Fourier maps.

90.2(5). Three selected sections of the reciprocal space were calculated (Figure 4a−c), and their indexations are consistent with a body-centered I type symmetry, confirming the results obtained from SAED. After the space group test carried out in JANA2006, the highest symmetry space group Im3̅m was selected for the structure determination since it presented the best redundancy. Among the 2807 measured reflections, 157 independent reflections were retained with I > 3σ leading to Rint of 36.13 and a redundancy of 17.8. After three cycles of calculations, SUPERFLIP found a solution with a cumulative coverage of 100% and a dmin resolution value of 1 Å. The overall agreement 6406

DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409

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Inorganic Chemistry

Figure 7. (a) Structural model of Ba19Cr12O45, (b) Cr1 and Cr2 octahedra with their sharing face highlighted by red dashed lines.

Table 2. Cell Parameter, Atomic Positions, and Interatomic Distances of Ba19Cr12O45 (Im3̅m) from the Rietveld Refinement of the Powder XRD Patterna Ba19Cr12O45(Im3̅m) (Z = 2) cell parameter a (Å)

13.3103(6)

atomic positions

x

y

z

Wyckoff site

Ba1 Ba2 Ba3 Cr1 Cr2 O1 O2 O3 O4 impurities

0.2980(4) 0.2980(4) 0 24h 0.3088(9) 0 0 12e 0 0 0 2a 0.1378(9) 0.1378(9) 0.1378(9) 16f 0.25 0.25 0.25 8c 0.158(3) 0 0.158(3) 24h 0.711(2) 0.153(2) 0.153(2) 48k 0.5 0 0 6b 0.5 0.25 0 12d -Ba3(CrO4)2 (16%, R3̅m) -Hexagonal Ba-based phase (P63/mmc not quantified) -Unknown phase (not identified) interatomic distances (Å) multiplicity

Figure 8. XRD pattern at ambient conditions of the Ba2CrO4 sample after its HP-HT treatment with in red the experimental pattern, in black the calculated one (by Rietveld refinement), and in blue the difference. From the top to bottom, Bragg reflections are assigned to their corresponding phase: the main phase: Ba19Cr12O45 (Im3̅m), and impurities: Ba3(CrO4)2 (R3̅m) and an hexagonal Ba-based phase (P63/mmc) observed in TEM experiment (Le Bail refinement).

Ba1−O1 Ba1−O4 Ba1−O2 Ba1−O2 Ba2−O3 Ba2−O2 Ba3−O1 Cr1−O1 Cr1−O2 Cr2−O2

factor is 19.88, which might be high for X-ray data, but is a rather good value for ED data considering the remaining dynamical diffraction effects. The [100] projection of the electrostatic potential isosurface of the structure solution is presented in Figure 5a, and its interpretation by SIR2011 in term of atomic positions is presented in Figure 5b. The raw structure solution contains three independent Ba positions in 24h (Ba1), 12e (Ba2), and 8c (Ba3) Wyckoff sites, two Cr atoms in 16f (Cr1) and 2a (Cr2) sites, and three O positions in 48k (O1), 24h (O2), and 48j (O3) sites. This model gives rise to a general chemical formulation Ba44Cr18O120, with a ratio Cr/Ba = 0.33, which is much lower than the ratio Cr/Ba = 0.55 ± 0.15 evidenced by EDX analysis. It seems to indicate that Ba atoms may be overestimated and Cr atoms underestimated in this model. Furthermore, the O3 atomic position generates oxygen clusters in the structure, with O−O distances < 1.5 Å. To have a better view of the right chemical nature of the elements in the atomic sites, a study of the interatomic distances was carried

a

2.64(3) 2.764(5) 2.81(3) 2.96(3) 2.54(2) 2.89(3) 2.97(4) 1.87(4) 2.03(3) 1.90(3)

1 2 4 2 1 4 12 3 3 6

Biso values were fixed to 1 Å−2.

out (Table 1). Before interpreting the distances in Table 1, let us note the theoretical bond length from the Shannon tables: Ba−O distances ≈ 3 Å and Cr−O distances ≈ 2 Å.23 One can observe that Ba1 and Ba2 respect the expected distances with their surrounding oxygen atoms, but the Ba3−O1 bond length value of 2.00 Å suggests clearly that Ba3 is very likely a Cr atom. Analogously, given the bond lengths Cr1−O1 and Cr1−O2 close to 2 Å, there is no doubt that Cr1 is truly a Cr atom, but the distances of 2.85 Å between Cr2 and 12 O2 atoms are signs of Ba 6407

DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409

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Inorganic Chemistry

Cr2 site. Anyway, this potential disproportion has to be checked by complementary measurements in the future, like a spectroscopic one. On the basis of the model issued from the TEM study, the cubic Ba19Cr12O45 chromate structure was refined using the Rietveld method applied to modelize the experimental powder XRD pattern (Figure 8). The calculated positions and intensities of the Bragg reflections from Ba19Cr12O45 fit perfectly with the majority of the experimental peaks. In addition to the main phase, two impurities were taken into account to improve the fit. The first one is the hexagonal Ba-based phase evidenced in the TEM study, whose cell parameters were refined using the LeBail method (a = 10.444(2), c = 6.536(2), P63/mmc). The second phase, Ba3(CrO4)2 (R3̅m),25 was finally identified from the remaining weak XRD peaks and refined with the Rietveld method. In the end, the final refinement shows rather good agreement factors, even if some few weak peaks remain still non indexed (global χ2 = 13.8, RBragg = 7.68). The obtained cell parameter, atomic positions, and interatomic distances of the Babased cubic chromate are listed in Table 2. One note that the octahedron around Cr2 site is not distorted, while the one around Cr1 site is slightly distorted. This information could eventually be checked in the future by a neutron powder diffraction study to better determine the position of oxygen atoms. Finally, we report some of the physical properties of this new cubic Ba19Cr12O45 chromate. No transport measurements were performed due to the strong insulating behavior of the sample at ambient conditions. Figure 9 shows the temperature dependence of the magnetic susceptibility χ for Ba19Cr12O45 measured under a magnetic field of 1 T. No anomaly related to any long- or shortrange order of Cr magnetic moments is found, which is maybe related to the disproportion of Cr sites in Cr4+ and Cr5+ alternating sites. The presence at 16% of the AFM Ba3(CrO4)2 impurity phase does not lead to its (weak) AFM signature on raw data expected at TN = 15 K.26 The inset illustrates the inverse susceptibility 1/χ versus temperature typical of ferrimagnetism. The linear behavior of 1/χ at high temperature could be fitted with a Curie−Weiss law between 115 and 315 K and a paramagnetic Curie temperature θp = −55 K, which indicates antiferromagnetic interactions between Cr cations. The value of Peff = 2.36 μB obtained from the calculated Curie constant C = 0.787 emu·K·mol−1·Cr−1 corresponds to a proportion of 88% of Cr4+ (2.6 μB) and 12% of Cr5+ (1.7 μB), which is in rather good agreement with the 16/8 proportion found in the structural model determined by TEM.

Figure 9. Temperature dependence of the magnetic susceptibility of Ba19Cr12O45 measured in a magnetic field of 1 T. Inset: the inverse of the magnetic susceptibility versus temperature with a Curie−Weiss fit in red.

atom instead of Cr. After redistribution of Ba and Cr atoms in their proper crystallographic sites (Cr2 → Ba3 and Ba3 → Cr2), it has been decided to delete O3, giving too short interatomic distances (O3−O3 < 1.5 Å and Ba2−O3 = 2.41 Å). The chemical formula becomes Ba38Cr24O72, with a ratio Cr/Ba = 0.63 in agreement with the one from EDX analysis. To determine the missing oxygen atoms, the structure was refined kinematically by JANA2006, and difference Fourier maps were computed. Among the possible positive remaining peaks, two of them with reasonable interatomic distances were retained as possible oxygen positions. They were labeled O3 and O4, and they gave interatomic distances with their closest neighbors atoms O3−Ba2 = 2.56 Å and O4−Ba1 = 2.8 Å. These two oxygen atoms are located on the faces of the cubic unit cell, as illustrated by the section z = 0 of the electron density map in Figure 6. The final structural model in Figure 7a exhibits two Cr sites both in octahedral environments, with face sharing octahedra between Cr1 and Cr2 (highlighted by red dashed lines in Figure 7b), and corner-sharing octahedra between two Cr1. The whole unit cell can be described as a repetition of eight formula groups Ba4.75Cr3O11.25. Normalized to Cr, the chemical formula becomes Ba1.58CrO3.75, which shows that this Ba-based chromate has a stoichiometry different from the one of the n = 1 RP member, namely, Ba2CrO4, in agreement with the presence of secondary phases such as the Ba-rich phase detected in EDX analysis. Furthermore, this formulation gives an average Cr valence of 4.33, which suggests that Cr1 sites (16f) are occupied by Cr4+ and Cr2 sites (8c) are occupied by Cr5+, i.e., maybe a charge disproportion. To support this estimated valencies, bond valence sum (BVS) calculations were performed using bond valence parameters compiled by Gagné and Hawthorne24 and bond distances. If bond valence parameters for Cr4+ are used, a BVS of 3.95 was obtained for Cr1 site. Cr5+ is not so easy to check, as bond valence parameters determined by Gagné and Hawthorne used only one datum for this state. When these parameters for Cr5+ are used, a BVS of 4.3 was calculated, which is not close to 5, but this number should be viewed with care, as noted before. When Cr3+, Cr4+, and Cr6+ (parameters for those are based on more data) are assumed, 3.7, 4.5, 4.6 are calculated, respectively, which seems to indicate that Cr3+, Cr4+, and Cr6+ are not likely for

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CONCLUSION We have discovered a Ba-based chromate through an HP−HT solid state synthesis. It exhibits an original cubic lattice with a Ba19Cr12O45 chemical composition. Its crystal structure was solved by electron diffraction tomography and refined using powder X-ray diffraction data. The structure exhibits a threedimensional network of CrO6 octahedra built around two Cr sites, Cr1 and Cr2, with face sharing between respective Cr1 and Cr2 octahedra and corner sharing between adjacent Cr2 octahedra. The chemical formula, local distortion of each kind of Cr octahedron, and BVS of Cr1 and Cr2 sites are suggestive of a charge disproportion between Cr4+ and Cr5+ localized in Cr1 and Cr2 sites. This new oxide is an electrical insulator at ambient temperature and does not order magnetically at low temperature. In addition, the magnetization measurement shows AFM 6408

DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409

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Inorganic Chemistry

(13) Drozhzhin, O. A.; Sumanov, V. D.; Karakulina, O. M.; Abakumov, A. M.; Hadermann, J.; Baranov, A. N.; Stevenson, K. J.; Antipov, E. V. Switching between solid solution and two-phase regimes in the Li1‑xFe1‑yMnyPO4 cathode materials during lithium (de)insertion: combined PITT, in situ XRPD and electron diffraction tomography study. Electrochim. Acta 2016, 191, 149−157. (14) Palatinus, L.; Brázda, P.; Boullay, P.; Perez, O.; Klementová, M.; Petit, S.; Eigner, V.; Zaarour, M.; Mintova, S. Hydrogen positions in single nanocrystals revealed by electron diffraction. Science 2017, 355, 166−169. (15) Scholder, R.; Sperka, G. Ü ber Chromate (IV). Z. Anorg. Allg. Chem. 1956, 285 (1−2), 49−60. (16) Young, D. S.; Sachais, B. S.; Jefferies, L. L. C. Rietveld Method 1993. (17) Roisnel, T.; Rodríguez-Carvajal, J. WinPLOTR: a windows tool for powder diffraction pattern analysis. Mater. Sci. Forum 2001, 378-381, 118−123. (18) Palatinus, L. PETS - Program for Analysis of Electron Diffraction Data; Institute of Physics of the ASCR: Prague, 2011, http://pets.fzu. cz/. (19) Palatinus, L.; Corrêa, C. A.; Steciuk, G.; Jacob, D.; Roussel, P.; Boullay, P.; Klementová, M.; Gemmi, M.; Kopeček, J.; Domeneghetti, M. C.; Cámara, F.; Petřiček, V. Structure refinement using precession electron diffraction tomography and dynamical diffraction: tests on experimental data. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, 71, 740−751. (20) Palatinus, L.; Chapuis, G. SUPERFLIP-a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J. Appl. Crystallogr. 2007, 40, 786−790. (21) Petříček, V.; Dušek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General features. Z. Kristallogr. - Cryst. Mater. 2014, 229 (5), 345−352. (22) Cao, L. P.; Jin, M. L.; Li, W. M.; Wang, X. C.; Liu, Q. Q.; Xu, Y. L.; Pan, L. Q.; Jin, C. Q. High-pressure and high-temperature synthesis and physical properties of Ca2CrO4. AIP Adv. 2016, 6, 055010−7. (23) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (24) Gagné, O. C.; Hawthorne, F. C. Comprehensive derivation of bond-valence parameters for ion pairs involving oxygen. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, 71, 562−578. (25) Mattaush, H.; Müller-Buschbau, H. On the Crystal Structure of Ba3(CrO4)2. Z. Naturforsch. 1972, 27b, 739−744. (26) Nakajima, T.; Mitamura, H.; Ueda, Y. Singlet ground state and magnetic interactions in new spin dimer system Ba3Cr2O8. J. Phys. Soc. Jpn. 2006, 75 (5), 054706.

correlations and seems to be in agreement with the charge disproportion scenario.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1547718 contains the supplementary crystallographic data for this paper (the CIF file was generated by Fullprof program after Ba19Cr12O45 structure refinement). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christophe Lepoittevin: 0000-0002-3995-0746 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.J. thanks the LANEF framework with mutualized infrastructure (ANR-10-LABX-51-01) for the financial support during his Ph.D. work. The authors acknowledge C. Goujon and M. Legendre for their help in the use of the Conac large volume press during the HP-HT synthesis of our samples.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00481 Inorg. Chem. 2017, 56, 6404−6409